The current project will identify maturational changes in the infant brain that accompany important transitions in social behavior in the first 6 months o life. This work is motivated by a surprising feature of a recent study in which we identified the earliest known indicators of social disability in autism (Jones & Klin, Nature, 2013). The data falsified a prior hypothesis by revealing that eye-looking-a basic mechanism of social adaptive action in typical infants-was not immediately diminished in infants with autism; instead, infants with autism exhibited a slight but significant increase in eye-looking at 2 months, which then declined; in contrast, typical infants exhibited a relative low point in eye-looking at 2 months, which then increased. The timing of this difference highlights a narrow developmental window, around month-2, that spans an important period of dynamic transition in typical infants. At approximately 4-6 weeks, reflex-like neonatal predispositions-including among others: non-social, endogenous smiling1, 2; reflexive imitation3-5; and reflexive attention to faces6-decline. This decline is followed by major changes in infant behavior: after month 2, infants begin to engage in contingent social interaction: they increase time spent looking at others' eyes10,11 and produce socially-elicited smiles during interaction with caregivers2,12,13. The neural mechanisms supporting these critical transitions are thought to be a shift from subcortical to cortical control1,6,8,14-16, with initial reflex- like predispositions (subserved by subcortical structures) declining as cortical control matures. This account appears well-fitted by our eye-tracking data11, and suggests a very specific hypothesis in autism: in infants with autism, reflex-like (subcortically-mediated) orientation to the eyes of others may initially be intact, while the emergence of experience-dependent (cortically-mediated) voluntary eye-looking subsequently fails. Despite the appeal of this proposed model, no studies have prospectively tracked the development of these neural systems and their relationship to unfolding behavior in typical development, a critical step before examining disruptions thereof in autism. In N=50 typical infants we will examine developmental changes in brain connectivity associated with 2 critical transitional behaviors, eye-looking and social-smiling, to identify: (1) brain networks differentialy associated with either decline in reflexive eye-looking or with the emergence of voluntary eye-looking, (2) brain networks differentially associated with either the decline of endogenous, reflexive smiling or with the emergence of socially-elicited smiling, and (3) brain networks common to social adaptive action. Behavioral and neuroimaging measures will be collected longitudinally at 7 and 3 time points, respectively, between birth and 6 months. Brain connectivity will be measured both structurally (as tract- specific diffusion parameters, including fractional anisotropy (FA) and mean diffusivity (MD), via diffusion tensor imaging (DTI) and tractography) and functionally (via measures of resting-state functional connectivity).